Containers for storing or transporting powdered or granular commodities frequently employ pneumatic conveyors to facilitate movement and discharge of the commodity. Gas pervious membranes, such as canvas or other fabrics, are commonly used to diffuse and direct the flow of fluidizing gas into the commodity. However, such membranes require complicated and costly mechanical devices to hold them in place, and they can not be cleaned simply by hosing down with water. Also, they are frequently clogged by powdered commodities or the fines of granular materials. This sometimes prevents their use in railroad vehicles. Prior art pneumatic conveyors using gas impervious membranes possess serious deficiencies when fluidizing gas flow has to be accurately controlled for long distances, or has to be readily changeable for different commodities or conditions.